The present invention relates generally to a method of using sacrificial materials for fabricating internal cavities and channels in laminated dielectric structures, which can be used as dielectric substrates and package mounts for microelectronic and microfluidic devices.
Multilayered, laminated dielectric structures are commonly used in microelectronic and microfluidic devices for substrates and package mounts. Multilayered dielectric materials include printed wiring board (PWB) laminate materials, low-temperature co-fired glass/ceramic dielectric materials (LTCC) and high-temperature co-fired ceramic/glass (HTCC) materials.
For storing and operating microelectronic devices in severe environments, ceramic packages are preferred because they are generally stronger and more hermetic than plastic encapsulated packages. These packages are typically made by laminating multiple layers of unfired glass/ceramic sheets. Individual layers of unfired glass/ceramic dielectric material (i.e., green ceramic) are created by casting a blend of ceramic and glass powders, organic binders, plasticizers, and solvents into sheets or tapes (e.g., tape casting). The organic components provide strength and flexibility to the green/unfired sheets/tapes during handling. The tape-cast sheets (i.e., “green tape”) are then personalized by cutting out individual sheets having the appropriate outside perimeter and internal apertures (if needed). This can be done by punching, water-jet cutting, laser cutting, conventional machining, etc. Lines or traces of conductive metallized pastes or inks can then be deposited on individual sheets by using a thick-film screen-printing process, MicroPen™ direct-write printing, etc. Thick-film resistors can also be deposited at this stage. Transverse vias can be machined into selected sheets by punching, drilling, or laser ablation. The vias are then filled with a conductive paste by screen-printing, which allow conductive traces on one level to be electrically connected to conductive traces on another level. The individual sheets are then collated, stacked, and laminated. Optionally, an adhesive or solvent may be applied to the individual sheets prior to lamination. Then, the stack is then vacuum bagged and isostatically compressed at high pressure (e.g., 3000 psi) and moderate temperatures (e.g., 60-80 C) until the individual sheets adhere to form a laminated assembly. Uniaxial pressing, with pressures as high as 30,000 psi, can also be used.
Next, the laminated assembly is baked at relatively low temperatures (e.g. 350-450° C.), to remove organic binders and plasticizers from each layer and from conductor/resistor pastes. After this “burnout” step has been completed, the assembly is fired at much higher temperatures (e.g., 850 C for LTCC, up to 1650 C for HTCC), which sinters and densifies the glass-ceramic substrate to form a dense, rigid, monolithic ceramic structure. During firing, glass-forming constituents in the layers flow and advantageously fill-in voids, corners, etc. Because all of the individual sheets of tape-cast ceramic material (including patterned lines of conductive pastes (Au, Ag) and filled vias) are baked and fired simultaneously, the product made by this process is conventionally designated as a co-fired ceramic dielectric material.
Two different co-fired ceramic multilayer systems are commonly used, depending on the choice of materials and processing temperatures: high-temperature co-fired ceramic (HTCC), and low-temperature co-fired ceramic (LTCC). When the ratio of ceramic-to-glass is high (e.g., 9-to-1, or greater), the green tape can only be sintered (e.g. densified) at very high firing temperatures (e.g. 1300 to 1800 C). Accordingly, thick-film conductive pastes for HTCC systems comprise high-melting point metals, such as tungsten, or alloys of molybdenum and manganese. The dielectric consists of glass fillers in a ceramic matrix. During firing a glassy phase is formed from the presence of various oxides in the ceramic.
Alternatively, in the LTCC system, the dielectric can comprise a ceramic-filled glass matrix, which is typically sintered at much lower firing temperatures (e.g. 600 to 1300 C, typically around 850 C). Thick-film metallization can use high-conductivity metals, such as gold, silver, copper, silver-palladium, and platinum-gold.
Hereinafter, when the term “LTCC” is used, it is intended to also include the HTCC material system, unless specifically stated otherwise.
Internal cavities, recessed volumes, microchannels, etc. are used in microelectronic and microfluidic devices to provide spaces for mounting Integrated Circuit (IC's) or other discrete devices, and/or for providing means for conveying and/or storing a liquid or gas within the device or from one location to another. These internal “cavities” are conventionally fabricated by cutting out a volume of material from a specific layer inside the stack of layers, thereby making an aperture, as shown in FIG. 1. However, if the dimension of the cutout is much wider than it is tall, then during the lamination step the internal cavity walls above and below the cutout can sag or even collapse when high pressures are applied.
To solve this problem, the conventional approach is to use a temporary insert (made of a different material) placed inside of the cutout volume, as shown in FIG. 2. The temporary insert is sized to closely match the shape of the cutout volume and thickness of the layer (i.e., 2nd layer in FIG. 2), to provide adequate support against collapse of the internal cavity walls during lamination.
In U.S. Pat. No. 5,601,673, “Method of Making Ceramic Article with Cavity Using LTCC Tape”, Alexander teaches that it is important that the temporary insert have the same thickness as the ceramic tape layers to assure good dimensional control.
In the case where the temporary insert can not be simply picked or pulled out after the final firing step, then other methods must be used to remove the insert. Alexander (ibid) teaches that the insert can be made of a “fugitive” or sacrificial material that “burns out” and disappears during the burnout/firing step. Examples of fugitive materials taught by Alexander include: (1) the same materials as the binder/plasticizer system used in the green LTCC dielectric tape, and (2) a dough-like material comprising cornstarch, petroleum jelly, and a small amount of plasticizer. As illustrated in FIG. 3 of the instant application, the fugitive insert (sacrificial material) supports the neighboring cavity walls during lamination. Then, the sacrificial material burns out during the burnout step. Finally, the structure is fired to densify the ceramic, leaving an internal cavity that has not sagged or collapsed.
In U.S. Pat. No. 5,779,833, “Method for Constructing Three Dimensional Bodies from Laminations”, Cawley et al. disclose the use of fugitive materials for providing temporary support of internal cavities, where the thickness of the fugitive layers is chosen to allow the forming of a flat surface coplanar with the surface of the subassembly after each layer is stacked. Examples of fugitive materials taught by Cawley et al. include: (1) acrylic latex in a colloidal suspension; (2) walnut flour made by grinding walnut shells and organic gels; (3) corn starch suspended in an aqueous slurry or suspended in toluene or polyvinyl butyrate; and (4) inorganic oxide ceramic powders greater than 3 micron diameter held in a polymer binder, which turns into a flowable powder after the binder has been burned away.
In U.S. Pat. No. 4,806,295, “Ceramic Monolithic Structure Having an Internal Cavity Contained Therein and a Method of Preparing the Same”, Trickett et al. teach the use of low-melting point paraffin wax or Woods metal to provide temporary support of internal cavity walls during the lamination step. After isostatic pressing, the laminated structure is placed in a suitable position and heated to allow the melted supporting media to “drain away” from the structure.
Alternatively, photoresist has been used to define internal channels. The temporary photoresist material is removed by exposing the photoresist to acetone, which dissolves the photoresist, and allows it to flow out of the channels.
The conventional method of using a temporary, matching insert to provide cavity support, as illustrated in FIGS. 2 and 3, has a number of problems and disadvantages. First, a cutout has to be made in the layer. Then, a closely matching insert has to be fabricated and inserted into the cutout. All of these steps require additional time and costs. When the internal cavity comprises complex-shapes or multiple-curved channels (e.g., serpentine channels), then the steps of making the cutout and fabricating a matching insert become even more expensive and time-consuming. At some level of complexity (of the cutout pattern), the costs of using complex shaped inserts is prohibitive.
What is needed, therefore, is an easier and less expensive method for fabricating internal cavities in multilayered dielectric structures; preferably that doesn't require the use of cutouts and matching inserts. Preferably, the improved method would be self-aligning and self-assembling.
Against this background, the present invention was developed.